Current limiting fuse

ABSTRACT

A current limiting fuse having a fusible element with a plurality of longitudinally spaced holes or slots or otherwise reduced cross section and utilizing in addition at least one point along the element a small quantity of a low melting point material, such as tin. The low melting point material and at least a portion of the holes or reduced sections is shunted by a length of high resistance, highly reactive exothermic wire. Since the exothermic wire is close to the main piece of fuse material, the proximate force of the explosion blows out or melts other sections of the main fusible element in the weakened areas near the holes or reduced sections to quickly limit any overcurrent.

[72] Inventor: v [73] Assignee: Westinghouse Electric Corporation,

'[52] U.S.Cl

United States Patent Cameron A [54] CURRENT LIMITING FUSE Frank L Ca nero n, Irwin, Pa.

Pittsburgh, Pa.

[22] Filed: May 24, 1971 [21] 7 Appl. No.: 146,207,

[ Dec. 5, 1972 604,102 6/1948 Great Britain ..337/293 Primary Examiner-Harold Broome v Attorney-A. T. St'ratton and Clement L. McHale [5 7] ABSTRACT A current limiting fuse having a fusible element with a plurality of longitudinally spaced holes or slots or otherwise reduced cross section and utilizing in addi- 51 Int. Cl. ..'L....'....'...II01h 85/04 time least One Point along the element a small-quan- [58]v Field of Search 337/158 159 160 290 293 tity of a low melting point material, such as tin. The low melting point material and at least a portion of the 337/295, 401 I holes or reduced sections IS shunted by a length of I high resistance, highly reactive exothermic wire. Since [56] References Cited the exothermic wire is close to the main piece of fuse UNITED STATES PATENTS material, the proximate force of the explosion blows out or melts other sections of the main fusible element ,24 1966' Mikulecky ..337/1 60 X in the weakened areas near the holes or reduced sec- 3,134,874 5/1964 Cameron "337/ 158 X tions to quickly limit any overcurrent.

FOREIGN PATENTS OR APPLICATIONS 6 Claims, 4 Drawing Figures 626,530 8/1961 Canada ..337/l60 Q 14 34- 7 "pi 47-\ M 3s v Q' PATENTEDnEc 5:912

FIG.|.

FIGZA.

. 1 CURRENT LIMITING FUSE BACKGROUND OF THE INVENTION This invention relates to electrical fuses and it has particular relation to current limiting fuses for interrupting relatively low overload currents. xv I Current limiting fuses typically include fusible elements formed from fusible ribbon material or wire of fusible material with axially spaced reduced cross-sectional areas. When the overload current is relatively high, such as when the corresponding rated or normal current flowing through the fuse is relatively high, the fusible elements of the fuse melt or disintegrate in the regions of the'reduced cross-sectional areas because this is where the current density is relatively highest. Consequently, ,multiplearcs are established between the various sections of fusible material which remain after the initial melting of the fusible elements. These arcs present a relatively high impedance to higher magnitudes of current flow which maybe available in the circuit to be protected. This is especially true if the circuit to be protected has a relatively high inductive component of reactance. Such being the case, the overload current is limited quickly to a value less than available in the circuit to be protected.

1 However, in relatively lowcurrent applications where a current limiting fuse is connected in a circuit in which relatively lowvalues of overload current flow, the amount of overload current necessary to melt or disintegrate one of the weakened areas is also relatively low. It is therefore not likely that all the weakened or reduced cross-sectional regions will melt or fusesimultaneously, but that one of them will melt first and as it melts, an are results which has a tendency to reduce or limit the flow of overload current. But a single are usually does not have the limiting effect on electrical current flow that a plurality of arcs has. Consequently, the available driving voltage of the protected circuit may force the current intended to-be interrupted to, in fact, be sustained or even increase for a short period of time and possibly damage or destroy associated electrical equipment.

In the past, it has been proposed, such as in US. Pat. No. 3,243,552 issued Mar.'29, 1966, to H. W. Mikulecky to provide adjacent to the main fusible element a section of fusible material which is formed from material similar to the fusible material of the main element. The adjacent section of fusible material is mounted proximate to the main fuse element, but is not attached or connected to it. Instead, electrically insulating gaps of appropriate length are provided between the ends of the adjacent section of fusible material and the main fuse element. When the mainfuse element melts or fuses to create an arc voltage and a larger impedance to the flow of overload current, there is a tendency, because of the latter arc voltage, for the overload current to flow through the gaps between the main fuse element and the adjacent section of fusible material at the respective endsfthereof, thus creatingadditional arcs. These arcs have a tendency to further reduce current flow, but only by a slightly larger amount, as usually only two more arcs are created and both of these are in parallel with the primaryarc.

It would be advantageous to create a relatively larger number of gaps and thus a larger number of current limiting arcs in a low current fuse.

SUMMARY OF THE INVENTION In accordance with the invention, a main length or element of fusible material which may be in the form of either wire or a ribbon strip includes a plurality of Iongitudinally spaced restricted portions by forming notches in the wire or holes or notches in the strip of fusible material.

In the case of a strip or ribbon of fusible material, a silver composition may be used to form the ribbon or strip. Holes may .then be formed at longitudinally spaced intervals which may be substantially equidistant or otherwise spaced from one another along the length of fusible material. Consequently, reduced cross-sectional areas result on either side of each hole or opening. In one embodiment, one of the holes may then be filled with a low melting point material, such as a tin composition. In another embodiment, a similar tin composition may be deposited in the form of an adhering bead to the main fuse material in a spaced relationship between holes or notches. The fusible material will melt initially at the'tin head or tin filled hole when subjected to a predetermined overload current. However, a portion of the fusible material, which may include the low melting point material, is paralleled or shunted by a strip of-exothermic or high explosive fuse wire, such as that sold under the trademark Pyrofuze. Pyrofuze material is'of such a composition that when the current through it increases significantly, it will melt with explosive force. The strip of exothermic or explosive wire is disposed adjacent to the main fuse element and electrically connected in parallel with at least a portion of said element. The resistance of the explosive wire is much higher than that of the main fuse element, and consequently when the main fuse element is conducting normal or less than a predetermined or rated current, most of the current flows through the relatively lower resistance of the main fuse element. However, as the relatively low current increases to an overload value, the main fuse element will melt or fuse in a re-" gion of restricted area or of the low melting point material creating an arc and increasing the effective impedance or resistance of the main fuse element when compared with that of the auxiliary element. Consequently, a significant amount of the overload current will then flow through the shunt or auxiliary element, causing it to explode and exert an exothermic explosive force though granular or pulverulent arc quenching material such as silica sand upon the adjacent or proximate portion of the main fuse element. When this happens, the restricted portions of the main fuse element near the remaining holes are blown away or melted by the hot explosive gases from the exothermic or explosive wire. This has the effect of quickly creating a plurality of substantially simultaneously produced arcs which quickly limit current flow in the protected cir- Cult.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding .of the invention, reference may be had to the preferred embodiments exemplary of the invention shown in the accompanying drawings in which: I

FIG. 1 is a view of fuse element assembly including a shunting fuse wire;

FIG. 2 is a view of a fuse element assembly including a shunting fuse wire made of two different materials;

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and FIG. 1 in particular, a current limiting fuse element assembly capable of interrupting and limiting electrical current, by establishing voltage arcs is shown. Fuse element assembly 10 is adapted to interrupt relatively low values of overload current, that is overload current where the normal rated current of the fuse element assembly is relatively low. Fuse element assembly 10 comprises a first or main fuse element or ribbon 12. Fuse element 12 may be formed from silver or silver alloy suitable for fusing under predetermined overload current conditions. First fuse element or ribbon 12 includes a plurality of spaced areas or regions of reduced cross-section. In one embodiment of the invention, the regions 'of reduced cross-section are formed by removing generally circular portions of material from within the periphery of the fuse ribbon 12 at a plurality of longitudinally spaced intervals, thus forming a plurality of holes or openings as indicated at l4, l6, 18, 20, 22, and 24 in FIG. 1.

t In addition, a second or auxiliary, adjacent or proximate fuse element formed from a fusible material 28, which is exothermic or explosive and which may be wire, is connected in parallel with or shunts at least a portion of the main fuse element 12. The ends of the auxiliary fuse element 28 are attached or connected to main fuse element 12 at attachment points or connections 30 and 32.

. Areas of weakened or reduced cross-section as indicated at 34, 36, 38, 33, 42 and 44 are formed near holes 14,16, 18, 20, 22 and 24, respectively. When normal or rated current 45 flows or less than normal rated current flows, most of'it flows through main fuse element 12 since the'auxiliary element 28 represents or forms a highly resistive shunt current path.

It will be noted that hole may be substantially filled with a low melting point material 26, such as a tin composition. This makes the main fuse element 12 especially weak in the region of hole 20 in the sense that when an overcurrent 45 flows, the weakened region 33 which comprises a first current conducting path 13 of main fuse element 12 will be the first to disintegrate, and creates an are between the remaining adjacent sections 12A and 12B of main fuse section 12. It is also possible for the fusible material in the region of hole 20 to melt first because of heat build up in the restricted region around hole 20 even if no low melting point material 26 is present. When melting occurs, the generated arc creates a greater impedance to the flow of current 45 through main fuse element 12, and consequently, there is a greater tendency for current component 47 to flow in adjacent, proximately disposed shunt wire or second fuse element 28, which comprises a second current conducting path 29. The explosive wire 28 may be made with a single concentrated core of aluminum which has an outer layer or jacket of palladium, forming an exothermic alloy. As the shunted overload current 47 begins to flow through exothermic wire 28, wire 28 begins to heat. In one embodiment of the invention, when the temperature reaches 660C, which is the melting point of aluminum, the inner concentric aluminum core melts causing a sudden exothermic alloying action which produces a great amount of heat. The heat may be released suddenly in the form of an explosion of hot gases which impacts upon the adjacent fuse section 12and blows away or disintegrates or melts the weakened sections 34, 36, 38, 42 and 44, for example. When this happens, additional arclets which are cumulative in nature form in the region of holes l4, l6, 18, 22 and 24 in addition to the original arc which initially occurred in the region of hole 20, thus creating a significant total are voltage along the length of the main fuse element 12. This operation tends to quickly reduce the effect of electrical momentum such' as may be found in a highly inductive protected circuit and to limit the magnitude of the current flow 45 to a value which is typically much lower than that which would otherwise result.

Referring now. to FIG. 2, a fuse element assembly 10 is shown in a second embodiment of the invention. Those parts of fuse element assembly 10 which are similar to those of fuse element assembly 10 as shown in FIG. 1 are indicated by the same numeral as used in.

FIG. 1, but with a prime added. In this embodiment of the invention, the auxiliary fuse element which comprises a section 50 of exothermic or explosive wire is connected or abutted at electrical junctions or connections 51 and 53 to sections 57 and 59 respectively of highly resistive electrically conducting material, such as nickel-chrome or copper-nickel wire. The explosive reaction caused in fuse element section 50 is similar to that caused in fuse section 28, as shown in FIG. 1. But less of the exothermic or explosive fuse wire, which may be expensive, is required because of the electrically conducting end segments of the previously mentioned highly resistive sections 59 and 57 which are connected to the main fuse element 12 as indicated at 32' and 30', respectively.

Referring now to FIG. 2a, a fuse element assembly 10" similar to the fuse element assembly 10' as shown in FIG.- 2, is depicted. Here, a bead of highly fusible tin composition or similar material 27 is disposed upon the surface of fuse ribbon or material 12'. Material 27 when heated burns away or fuses a portion of fuse material 12' at region 80, consequently causing a reaction similar to those previously described with respect to FIGS. 1 and 2.

Referring now to FIG. 3, an embodiment of the invention is shown incorporated into a cartridge type fuse 60. Cartridge type fuse 60 has oppositely disposed electrically conducting end sections or terminals 61 and 62 between which fuse element assembly 10 may be electrically connected and disposed. Fuse element assembly 10 may be enclosed by an electrically insulating or dielectric tube or housing 64 at the ends of which are placed or disposed end sections 61 and 62. Attached or secured to end sections .61 and 62 are electrically conducting ferrules or end caps 66 and 68, respectively, which may be used to connect cartridge fuse 60 to an associated fuse holder (not shown). As a result, the current limiting properties discussed with respect to FIGS. 1, 2 and 2a may be adapted to a cartridge fuse. A

pulverulentarc quenching material 72, such as silica sand, may be disposed within chamber 64 and around fuse element 10.

I It is to be understood that fuse element assembly 10, although illustrated as including a ribbon type main fuse element with longitudinally spaced relatively circular'holes, may include agenerally cylindrical main fuse element of the ,wire type which may also be tapered. It is also to be understood that the low melting point material provided to initiate the melting of the main fuse element during overload conditions may be formed from anysuitable material, such as a cadmium based material, which will electrically weaken oneregion of the fuse section more than other sections, and thus cause that section to break down or blow more path increases, said at least partial melting forming at least one region of reduced cross-section in said first fusible element and consequently increasing electrical resistance therein, said second fusible element being readilythan' any other sections. lt should also be understood that any number of holes or notched sections may be provided and spaced between connecting points of the auxiliary fuserlt is also to be understood that fuse element 28 may be attached in more places than the two ends, as illustrated in FIG. 1, to main fuse element 12. It is also to be understood that fuse section 10 need not be limited to a silver composition fuse material nor need the wire or fuse element 28 be limited to any particular exothermic fusible material. It is to be further understood that the fuse element assem blies 10' or 10 may alternately be employed in the current-limiting fuse structure shown in FIG. 3 instead of the fuse element assembly 10. It is to be understood that although FIG. 3 shows fuse element assembly 10 incorporated in a cartridge type fuse, fuse element assembly 10 may be incorporated in many types of fuse structures and is not limited to only low current application, but may be used in a high current fuse application ifso desired. It is also to be understood that a fuse element assembly such as 10" may be formed in the shapeof a'spiral. It is also .to be understood that the bead or deposition of highly fusible material 12 may be used on section 12 as shown in FIG. 1 in place of or in addition to the'fuse material 26, which is formed in hole20. i I

The apparatus embodying the teachings of this invention has severaladvantages. For example, the applicants invention facilitates the construction of a current limiting fuse for effective clearing of low currents by using a smaller number of fuse elements of larger crosssectional area. It also provides a relatively simple way of creating a relatively large number of neat simultaneous arclets in a low rated current limiting fuse.

I claim as my invention: 4

'1. A current limiting fuse structure comprising a generally tubular, electrically insulating casing, a pair of terminals one disposed adjacent to each ofthe opposite ends of said casing, first and second fusible elements disposed within said casing proximate to one another with at least said first fusible element being electrically connected between said pair of terminals, said second fusible element being electrically interconnected with said first fusible element at at least two spaced locations, so as to provide first and second parallel electrically conducting paths adapted to carry current between said terminals, said first fusible element being adapted to at least partially melt between said spaced locations before said second fusible element begins to melt as electrical current in said first formed from a material having a relatively higher electrical resistance than said first fusible element, said second fusible element being adapted to suddenly release a quantity of heat when heated to a predetermined temperature, after said first fusibleelement partially melts the current in said second path increasing substantially due to said increased electrical resistance in said first path to thereby heat said second fusible element to said predetermined temperature and to thereby actuate said second fusible element to release said quantity of heat causing portions of said proximate first fusible element comprising said first current carrying path to melt from exposure to said heat to thereby establish at least one region between unmelted portions of said first fusible element between which an electric arc is struck while said current continues to flow which arc further limits the current flow in said current limiting fuse until said current issubstantially zero. g

. 2. The combination as claimed in claim 1, wherein said first fusible element comprises one or more'regions of relatively narrow cross-sectional area at least one of which is disposed between said spaced locations, said initial partial melting of said first fusible element beginning within at least one of said at least one or more regions between said spaced-locations as the current therethrough increases.

3. The combination as claimed in claim 2, wherein said second fusible element is formed from a generally exothermic explosive fuse material, said one or more regions of relatively narrow cross-sectional area being disposed in said first fusible element between said two locations at which said second fusible element is connected to said first fusible element.

4. The combination as claimed in claim 3, wherein said first fusible element is formed from ribbon-like substantially silver fusible material, said one or more regions including a plurality of relatively narrow crosssectional areas longitudinally spaced from one another and being formed by a plurality of holes, a first of said regions including at least one of said holes having affixed thereto low melting point composition material, said composition being adapted to melt as said current flowing in said first current path increases, said first fusible element bent adapted to initially melt upon flow of overload current in the said first region containing said low melting point composition, melting of the latter composition resulting in a high impedance arc voltage in said first region, said second fusible element being formed from explosive fuse wire, the current in said explosive fuse wire of said second fusible element increasing when said first fusible element melts to cause said explosive fuse wire to explode and release said quantity of heat, said explosion causing the remainder of said first fusible element to further melt in the regions of at least some of said plurality of holes, thus establishing a plurality of arcs, said plurality of arcs being effective to limit the current flowing in said fuse. t

5. The combination as claimed in claim 4 wherein said low melting point composition material comprises substantially a tin base.

'6. The combination as claimed in claim 4 wherein said low melting point composition material comprises substantially a tin base and is affixed within the associated hole within said first region in contact with said silver fusible material. 5 

1. A current limiting fuse structure comprising a generally tubular, electrically insulating casing, a pair of terminals one disposed adjacent to each of the opposite ends of said casing, first and second fusible elements disposed within said casing proximate to one another with at least said first fusible element being electrically connected between said pair of terminals, said second fusible element being electrically interconnected with said first fusible element at at least two spaced locations, so as to provide first and second parallel electrically conducting paths adapted to carry current between said terminals, said first fusible element being adapted to at least partially melt between said spaced locations before said second fusible element begins to melt as electrical current in said first path increases, said at least partial melting forming at least one region of reduced cross-section in said first fusible element and consequently increasing electrical resistance therein, said second fusible element being formed from a material having a relatively higher electrical resistance than said first fusible element, said second fusible element being adapted to suddenly release a quantity of heat when heated to a predetermined temperature, after said first fusible element partially melts the current in said second path increasing substantially due to said increased electrical resistance in said first path to thereby heat said second fusible element to said predetermined temperature and to thereby actuate said second fusible element to release said quantity of heat causing portions of said proximate first fusible element comprising said first current carrying path to melt from exposure to said heat to thereby establish at least one region between unmelted portions of said first fusible element between which an electric arc is struck while said current continues to flow which arc further limits the current flow in said current limiting fuse until said current is substantially zero.
 2. The combination as claimed in claim 1, wherein said first fusible element comprises one or more regions of relatively narrow cross-sectional area at least one of which is disposed between said spaced locations, said initial partial melting of said first fusible element beginning within at least one of said at least one or more regions between said spaced locations as the current therethrough increases.
 3. The combination as claimed in claim 2, wherein said second fusible element is formed from a generally exothermic explosive fuse material, said one or more regions of relatively narrow cross-sectional area being disposed in said first fusible element between said two locations at which said second fusible element is connected to said first fusible element.
 4. The combination as claimed in claim 3, wherein said first fusible element is formed from ribbon-like substantially silver fusible material, said one or more regions including a plurality of relatively narrow cross-sectional areas longitudinally spaced from one another and being formed by a plurality of holes, a first of said regions including at least one of said holes having affixed thereto low melting point composition material, said composition being adapted to melt as said current flowing in said first current path increases, said first fusible element bent adapted to initially melt upon flow of overload current in the said first region containing said low melting point composition, melting of the latter composition resulting in a high impedance arc voltage in said first region, said second fusible element being formed from explosive fuse wire, the current in said explosive fuse wire of said second fusible element increasing when said first fusible element melts to cause said explosive fuse wire to explode and release said quantity of heat, said explosion causing the remainder of said first fusible element to further melt in the regions of at least some of said plurality of holes, thus establishing a plurality of arcs, said plurality of arcs being effective to limit the current flowing in said fuse.
 5. The combination as claimed in claim 4 wherein said low melting point composition material comprises substantially a tin base.
 6. The combination as claimed in claim 4 wherein said low melting point composition material comprises substantially a tin base and is affixed within the associated hole within said first region in contact with said silver fusible material. 